Marine Mammal Acoustics

Marine mammals produce a wide range of sounds that help them navigate, find food, and communicate. Many of these sounds can be used to identify animals by species or group:

• Mysticetes (baleen whales) make low-frequency pulses, moans, and whoops
• Odontocetes (toothed whales) make mid- to high-frequency whistles and echolocation clicks
• Pinnipeds (seals and sea lions) make low- to mid-frequency growls, trills, and barks

Since 2006, we have used non-invasive passive acoustic monitoring to study the sounds produced by marine mammals. We deploy hydrophones at sea and analyze the noises we record to help us answer a range of questions about marine mammals:

• Where, when, and how many marine mammals are present in an area?
• What kind of sounds do they make and why?
• What other sounds are they exposed to?
• What are they doing at different locations?
• When are they in areas where they may need protection?

Call Identification

Much of our work involves listening to different marine mammal calls and identifying them by species. Some calls indicate specific behaviors such as feeding or breeding. For example, sperm whales will make echolocation clicks to detect their prey; male humpback whales sing elaborate songs to attract mates or establish territory with other males.

We use a variety of hydrophones that have different "listening ranges.” To make our analyses more efficient, we process the data we record using acoustic detectors that are specialized for finding the calls of each species. They can distinguish characteristic mammal calls from background noises such as those made by other animals, wind, waves, and ship noise. Scientists can then analyze the detected calls in a spectrogram—a visual display of the sound—and identify which species are present and what they might be doing.

Noise Exposure

Sounds made by human activity contribute significantly to ambient ocean noise levels. The noise type and level can impact the ability of marine mammals to communicate underwater and limit our ability to hear them in our passive acoustic data.

Baleen whales such as fin, sei, blue, humpback, minke, and North Atlantic right whales, produce low-frequency sounds. Their frequency range overlaps with:

• Sounds produced by vessels
• Airguns towed behind ships for seismic surveys
• Pile driving for offshore wind energy turbine construction
• Echosounders

Toothed whales such as dolphins, beaked whales, and sperm whales, produce higher frequency sounds. These sounds overlap with pile driving and almost the entire frequency range of echosounders and high-resolution geophysical equipment used to survey sites for offshore energy exploration. These human-made sounds can harm marine mammals by damaging their auditory systems if they are too close to the sound source. These sounds can also mask their calls, making it difficult for them to hear each other. We ensure that human activities avoid and minimize impacts to these animals.

We conduct a range of ocean noise research projects with our collaborators to monitor these human-caused noise levels and how they may be affecting the habitats, behavior, and health of marine mammals.

Movement and Population Size

Understanding where marine mammals are located and moving throughout the ocean is critical for species conservation. Using passive acoustic monitoring to record, detect, and classify calls by species allows us to learn more about when and where each species occurs.

We deploy hydrophones to listen in critical habitats, understudied areas, and along marine mammal migratory routes. We can analyze the acoustic data at seasonal, monthly, daily, hourly, or minute levels to determine if a species is common or rare at each recording site. Comparing these analyses between regions and years can reveal trends in seasonal presence and migratory patterns over time.

Using passive acoustics to estimate population density (number of animals per unit area) can be challenging because researchers must know the call rate of the species. Some species such as the North Atlantic right whale have variable call rates depending on their behavior. Other species have consistent call rates which make it easier to use passive acoustics for density estimation. For example, foraging sperm and beaked whales produce consistent echolocation clicks. Despite the challenges of using passive acoustics to estimate population size and density, it is very useful for understanding where and when different species occur. All of the data we have analyzed for marine mammal presence can be viewed on our Passive Acoustic Cetacean Map.

Behavior

To study marine mammal behavior, we can track their fine-scale movements using multiple synchronized recorders. This can be done using arrays of bottom-mounted recorders or towed hydrophone arrays behind a ship. We can use these arrays to pinpoint an animal’s position in either two or three dimensions. For example, we can track the depth of a sperm whale foraging dive using their echolocation clicks recorded on a towed hydrophone array.

We also combine visual and acoustic data to help understand the calls we are detecting. This allows us to correlate call types to specific species. Shipboard human observers provide the acousticians with the identification, location, and behavior of groups of whales and dolphins. The acoustician then checks the incoming sounds from a towed hydrophone array and uses specialized software to locate the source of the sounds. If the sound location matches the location provided by the observers and no other species are observed, then that sound can be attributed to that species.

Real-Time Threat Mitigation

We use passive acoustic monitoring to detect marine mammals in near-real time to mitigate threats such as ship strikes. We collaborate with the Woods Hole Oceanographic Institution to deploy autonomous ocean gliders and moored surface buoys equipped with hydrophones. Acoustic data is transmitted from these devices every two hours via satellite and uploaded to the Robots4Whales website. We analyze these data to detect sei, fin, humpback, and North Atlantic right whales. The acoustic detections from the gliders and buoys reduce the time and money spent searching for whales at sea using conventional vessel and aerial surveys.

The real-time system is used for monitoring migratory routes, fishing grounds, and offshore wind energy areas. It also aids visual surveys and shipping lane monitoring to reduce ship strikes. When endangered North Atlantic right whales are detected, NOAA establishes a slow zone around the area to encourage vessel operators to slow to 10 knots or less. These real-time detections are critical to improving conservation efforts by providing scientists, industries, and the public with information on baleen whale presence.

Resources

• North Atlantic Right Whale Acoustic Data and Annotations
• Passive Acoustic Detections Reports: North Atlantic Right Whale
• Passive Acoustic Reporting System Templates
• Real-Time North Atlantic Right Whale Reporting Template